hier.7: Storage partitions are not filesystems

I recommended the incorrect text to the submitter.

MFC after: 3 days
Fixes: f47cbb29e1c2 (Add /dev/gpt)
Reported by: Mark Millard <marklmi@yahoo.com>
Reviewed by: imp, mhorne
Approved by: mhorne (mentor)
Differential Revision: https://reviews.freebsd.org/D49523